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There are a few things in a pilot's life that affects him as much as the METAR. The METAR can ground you or tell you that you're legal to try an approach. It's one of those few things which follows you on every flight. Since a pilot spends a fair amount of time reading these things, occasionally something odd pops up. And then it nags at you. An example of that is the METAR descriptor GR. Why does GR mean hail? Wouldn't you think they'd choose HL? Perhaps that's too close to HELL. And that might confuse pilots who frequently see SS (Sandstorms) DS (Dust Storms) and VA (Volcanic Ash) which I think of as forms of weather hell.
So I decided to look at the whole list of METAR descriptors. Here they are:
Now this list almost makes perfect sense. SH = Showers and FZ = Freezing. I bet it makes sense to you too. Did you notice there were a few missing from this list? I pulled out the ones that didn't make sense to me. Here's the list:
See what I mean? Why BR means mist is not intuitive. How does a pilot learn these odd METAR contractions? How do we get "shallow" for MI anyway? Lots of questions and google didn't give me any answers.
So, I did a fair amount of research on each of these METAR descriptors to determine the origin. Some of this was very easy to do with the various online dictionaries and translators. The ones like BC and MI, were not so easy to track down. I had to call everywhere to find the answers. I called ICAO in Montreal. I called the World Meteorological Organization (http://www.wmo.int/pages/index_en.html) in Switzerland. That was an expensive phone call (+ 41 22 730 81 11). I even tried the local NOAA office. After several days of research and phone calls, I came up with the answers for the METAR descriptors I didn't understand.
Anyway, here is the list of why these odd METAR descriptors mean what they do.
Bet you never thought you'd need to know these things did you.
Five Other Weird Things I Learned
If that wasn't enough, I was able to track down 5 other little nit picky things I've always wondered about. I'm sure you've wondered about these as well. Here are 5 other mysteries of the METAR resolved:
1. I learned that the descriptors MI, BC and PR shall be used only in combination with the letter abbreviation FG, for example MIFG = Shallow fog.
2. I learned that DR (low drifting) shall be used for dust, sand or snow raised by the wind to less than two metres above the ground. BL (blowing) shall be used to indicate dust, sand or snow raised by the wind to a height of two metres or more above the ground. DR and BL will only be used with DU, SA, or SN.
3. I learned that SH (Showers) and TS (Thunderstorm) shall only be used with RA, SN, PL, GS and GR. For example SHSN (Snow Showers) and TSGS (Hail/Snow Pellet Showers).
4. I learned that the descriptor FZ shall be used only in combination with the letter abbreviations FG, DZ and RA, for
example FZRA (Freezing Rain).
5. And finally, within 8km of an airport, the proximity qualifier VC shall be used only in combination with the letter abbreviations TS, DS, SS, FG, FC, SH, PO, BLDU, BLSA, BLSN and VA.